Method and system for reducing effects of undesired signals in an infrared imaging system

ABSTRACT

Effects of undesired infrared light are reduced in an imaging system using an infrared light source. The desired infrared light source is activated and a first set of imaging data is captured during a first image capture interval. The desired infrared light source is then deactivated, and a second set of image data is captured during a second image capture interval. A composite set of image data is then generated by subtracting from first values in the first set of image data corresponding second values in the second set of image data. The composite set of image data thus includes a set of imaging where data all infrared signals are collected, including both signals resulting from the IR source and other IR signals, from which is subtracted imaging in which no signals result from the IR course, leaving image data including signals resulting only from the IR source.

FIELD OF THE INVENTION

The present invention generally pertains to active infrared (IR)imaging, and more specifically, to removing IR signals received fromunintended and undesired IR sources in a region of interest to improvethe quality of imaging data resulting from the active IR source.

BACKGROUND OF THE INVENTION

The presence of undesired signals is a concern in the processing ofvirtually all electromagnetic signals. Even in a relatively simplesystem, such as a radio, a squelch control is often provided toattenuate signals below a certain magnitude, so as to avoid undesiredbackground static being audible when a signal of interest is not beingreceived. What constitutes undesired background static is for the userto judge, and the user can set the squelch control to limit theaudibility of received signals based on the user's judgment.

Automated signal processing systems, where a computer systemautonomously responds to input signals, present a more difficultproblem. Unlike the example of a squelch control noted above, where auser can adjust the squelch level based on experience and judgment, itis more difficult to program a computer system to automatically set alimit to differentiate between types of signals that are desirable andthose that are not. For example, computers respond well to unambiguousinput from keyboards, pointing devices, and similar input devices, butrespond less satisfactorily to voice commands. Anyone who has usedspeech recognition programs has experienced some difficulty when thecomputer fails to recognize something the user said, which happens moreoften if there is any background noise or other sounds that affect theauditory input perceived by the computer.

Computer vision arguably is a much more intricate problem than speechrecognition. If the computer must process too many visual signals or toobroad a range of visual signals, the input will more likely be misreadby the computer. On the other hand, if the computer suppresses too manyvisual signals, the computer also may misread visual inputs or ignoreintended visual inputs entirely.

Today, computer vision is becoming an increasingly important field infurthering the desire to make computers and their interfaces even moreuser friendly. For example, the MIT Media. Lab, as reported by BryggUllmer and Hiroshi Ishii in “The metaDESK: Models and Prototypes forTangible User Interfaces,” Proceedings of UIST10/1997:14-17,” hasdeveloped another form of “keyboardless” human-machine interface. ThemetaDESK includes a generally planar graphical surface that not onlydisplays computing system text and graphic output, but also receivesuser input by “seeing” and responding to an object placed against thegraphical surface. The combined object responsive and display capabilityof the graphical surface of the metaDESK is facilitated using IR lamps,an IR camera, a video camera, a video projector, and mirrors disposedbeneath the surface of the metaDESK. The mirrors reflect the graphicalimage projected by the projector onto the underside of the graphicaldisplay surface to provide images that are visible to a user from abovethe graphical display surface. The IR camera can detect IR reflectionsfrom the undersurface of an object placed on the graphical surface. By“seeing” and detecting a specially formed object or IR-reflected lightfrom an object disposed on a graphical display surface, the metaDESK canrespond to the contemporaneous placement and movement of the object onthe display surface to carryout a predefined function, such asdisplaying and moving a map of the MIT campus.

Others have been developing similar keyboardless interfaces. Forexample, papers published by Jun Rekimoto of the Sony Computer ScienceLaboratory, Inc., and associates describe a “HoloWall” and a “HoloTable”that display images on a surface and use IR light to detect objectspositioned adjacent to the surface.

Both the metaDESK and HoloWall/HoloTable use IR light to see objects andmovements for good reasons. If the systems responded to visible light,visible light projected by the systems and reflected back by theinteractive surface could lead to false readings by the computingsystem. Further, even if reflections could be suppressed, unless thesystem is disposed in a dark room, room lights and other visible lightpassing through the interactive display surface would substantiallyadversely affect the computer vision systems.

Using reflected IR light to detect objects placed on an interactivedisplay surface avoids much of the problems that would arise fromattempting to recognize the objects with ubiquitous visible light.However, although people are generally aware of the IR content of lightproduced by most sources, because it is not visible to the naked eye,ambient IR light signals that might adversely impact computer visionsystems also are very common. Incandescent lights, the sun, and avariety of other common sources generate IR light. These unintended IRsignals, just like unintended visible light signals, can provideundesired input to IR-sensitive computer vision systems. Band-pass typefilters can suppress visible light and other non-IR light, but they arenot helpful in separating IR light reflected from an object that is tobe detected from background IR light.

It is therefore desirable to filter, mask, or otherwise reduce theeffects of unintended and undesired IR light signals, to prevent IRlight vision systems from responding to extraneous IR light signals. Theeffect of the undesirable background IR light should be avoided whendetecting objects without requiring that an IR computer vision system beoperated in an environment that shields it from all background IRsources.

SUMMARY OF THE INVENTION

One of the more important functions of the present invention is toreduce the effects of undesired IR sources, including ambient sourcessuch as sunlight, incandescent light, and other IR sources, in an IRimaging system. Imaging data are captured both when the IR sourcecontrolled by the IR imaging system is activated and when it is not. Theimaging data collected when the controlled IR source is deactivated areimaging data based on undesired IR sources. Thus, by pixelwisesubtracting a set of imaging data collected when the IR source wasdeactivated, from a set of imaging data collected when the IR source wasactivated, the resulting composite set of imaging data should includeonly imaging data resulting from illumination generated by thecontrolled IR source.

One aspect of the present invention is thus directed to a method forreducing effects of undesired IR light sources in an imaging systemusing an IR light source. The IR light source is activated during afirst image capture interval, and a first set of imaging data iscaptured during the first image capture interval. The IR light source isthen deactivated, and a second set of image data is captured during thesecond image capture interval. A composite set of image data is thengenerated by subtracting from first values in the first set of imagedata corresponding second values in the second set of image data.

In accordance with one embodiment of the present invention, activationof the IR light source is controlled by an image capture device, suchthat an image capture signal generated by the image capture devicecauses the IR light source to be activated during the first imagecapture interval and deactivated during the second image captureinterval.

The IR light source is disposed on a first side of a light-permeablesurface. As a result, the IR light source directs IR light on a physicalobject disposed adjacent an opposite side of the light-permeablesurface. The image capture device, like the IR source, is disposed onthe first side of a light-permeable surface and is used to capture IRlight cast by the IR light source that has been reflected by thephysical object.

The image processing system uses the light reflected by the physicalobject in the composite set of image data to recognize a characteristicof the physical object. In this method, the first values in the firstset of image data represent an intensity of IR light captured for eachof a plurality of points across the first side of the light-permeablesurface during the first image capture interval, while the IR lightsource was activated. The second values in the set of image datarepresent an intensity of IR light captured for each of the plurality ofpoints across the first side of the light-permeable surface during thesecond image capture interval, while the IR light source wasdeactivated.

In one embodiment of the present invention, the composite image data aredetermined by the equation:D(x,y)=I _(ON)(x,y)−I _(OFF)(x,y)where:

-   -   x,y represents a coordinate location of a point on the first        side of the light-permeable surface;    -   I_(ON)(x,y) represents intensity of IR light detected during the        first image capture interval at point x,y;    -   I_(OFF)(x,y) represents intensity of IR light detected during        the second image capture interval at point x,y; and    -   D(x,y) represents the net intensity of IR light at point x,y        when the intensity of the IR light captured at the point x,y        during the second image capture interval is subtracted from the        intensity of the IR light captured at the point x,y during the        first image capture interval.

The composite set of image data generated is provided to an imageprocessing system. There, the IR light reflected by the physical objectis used to recognize a characteristic of the physical object.Furthermore, a projector is preferably positioned on the first side ofthe light-permeable surface and is usable to present images on theopposite side of the light-permeable surface. A physical object on theopposite side of the light-permeable surface can thus interact with animage presented thereon.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a functional block diagram of a generally conventionalcomputing device or personal computer (PC) that is suitable for imageprocessing for the interactive display surface as used in practicing thepresent invention;

FIG. 2 is a cross-sectional view of a table-type interactive displaysurface, illustrating internal components;

FIG. 3 is an isometric view of an embodiment of the table-typeinteractive display surface that is coupled to an external PC;

FIGS. 4A, 4C, and 4E each show an enlarged cross-sectional view of aportion of the display surface, with a hand adjacent to the displaysurface illuminated by IR light from a controlled IR source and/orambient IR light, while FIGS. 4B, 4D, and 4F show resulting imagescaptured from the display surface based upon reflected IR light from thehand;

FIG. 5 is a block diagram of a system for reducing the effect ofundesired IR sources according to an embodiment of the presentinvention; and

FIG. 6 is a flow diagram illustrating the logical steps for reducing theeffect of undesired IR illumination according to an embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Exemplary Computing System for Implementing Present Invention

With reference to FIG. 1, an exemplary system suitable for implementingvarious portions of the present invention is shown. The system includesa general purpose computing device in the form of a conventional PC 20,provided with a processing unit 21, a system memory 22, and a system bus23. The system bus couples various system components including thesystem memory to processing unit 21 and may be any of several types ofbus structures, including a memory bus or memory controller, aperipheral bus, and a local bus using any of a variety of busarchitectures. The system memory includes read only memory (ROM) 24 andrandom access memory (RAM) 25. A basic input/output system 26 (BIOS),containing the basic routines that help to transfer information betweenelements within the PC 20, such as during start up, is stored in ROM 24.PC 20 further includes a hard disk drive 27 for reading from and writingto a hard disk (not shown), a magnetic disk drive 28 for reading from orwriting to a removable magnetic disk 29, and an optical disk drive 30for reading from or writing to a removable optical disk 31, such as acompact disk-read only memory (CD-ROM) or other optical media. Hard diskdrive 27, magnetic disk drive 28, and optical disk drive 30 areconnected to system bus 23 by a hard disk drive interface 32, a magneticdisk drive interface 33, and an optical disk drive interface 34,respectively. The drives and their associated computer readable mediaprovide nonvolatile storage of computer readable machine instructions,data structures, program modules, and other data for PC 20. Although theexemplary environment described herein employs a hard disk, removablemagnetic disk 29, and removable optical disk 31, it will be appreciatedby those skilled in the art that other types of computer readable media,which can store data and machine instructions that are accessible by acomputer, such as magnetic cassettes, flash memory cards, digital videodisks (DVDs), Bernoulli cartridges, RAMs, ROMs, and the like, may alsobe used in the exemplary operating environment.

A number of program modules may be stored on the hard disk, magneticdisk 29, optical disk 31, ROM 24, or RAM 25, including an operatingsystem 35, one or more application programs 36, other program modules37, and program data 38. A user may enter commands and information in PC20 and provide control input through input devices, such as a keyboard40 and a pointing device 42. Pointing device 42 may include a mouse,stylus, wireless remote control, or other pointer, but in connectionwith the present invention, such conventional pointing devices may beomitted, since the user can employ the interactive display for input andcontrol. As used hereinafter, the term “mouse” is intended to encompassvirtually any pointing device that is useful for controlling theposition of a cursor on the screen. Other input devices (not shown) mayinclude a microphone, joystick, haptic joystick, yoke, foot pedals, gamepad, satellite dish, scanner, or the like. These and other input/output(I/O) devices are often connected to processing unit 21 through an I/Ointerface 46 that is coupled to the system bus 23. The term I/Ointerface is intended to encompass each interface specifically used fora serial port, a parallel port, a game port, a keyboard port, and/or auniversal serial bus (USB). System bus 23 is also connected to a camerainterface 59, which is coupled to an interactive display 60 to receivesignals form a digital video camera that is included therein, asdiscussed below. The digital video camera may be instead coupled to anappropriate serial I/O port, such as to a USB version 2.0 port.Optionally, a monitor 47 can be connected to system bus 23 via anappropriate interface, such as a video adapter 48; however, theinteractive display table of the present invention can provide a muchricher display and interact with the user for input of information andcontrol of software applications and is therefore preferably coupled tothe video adaptor. It will be appreciated that PCs are often coupled toother peripheral output devices (not shown), such as speakers (through asound card or other audio interface—not shown) and printers.

The present invention may be practiced on a single machine, although PC20 can also operate in a networked environment using logical connectionsto one or more remote computers, such as a remote computer 49. Remotecomputer 49 may be another PC, a server (which is typically generallyconfigured much like PC 20), a router, a network PC, a peer device, or asatellite or other common network node, and typically includes many orall of the elements described above in connection with PC 20, althoughonly an external memory storage device 50 has been illustrated inFIG. 1. The logical connections depicted in FIG. 1 include a local areanetwork (LAN) 51 and a wide area network (WAN) 52. Such networkingenvironments are common in offices, enterprise wide computer networks,intranets, and the Internet.

When used in a LAN networking environment, PC 20 is connected to LAN 51through a network interface or adapter 53. When used in a WAN networkingenvironment, PC 20 typically includes a modem 54, or other means such asa cable modem, Digital Subscriber Line (DSL) interface, or an IntegratedService Digital Network (ISDN) interface for establishing communicationsover WAN 52, such as the Internet. Modem 54, which may be internal orexternal, is connected to the system bus 23 or coupled to the bus viaI/O device interface 46, i.e., through a serial port. In a networkedenvironment, program modules, or portions thereof, used by PC 20 may bestored in the remote memory storage device. It will be appreciated thatthe network connections shown are exemplary and other means ofestablishing a communications link between the computers may be used,such as wireless communication and wide band network links.

Exemplary Interactive Surface

In FIG. 2, an exemplary interactive display table 60 is shown thatincludes PC 20 within a frame 62 and which serves as both an opticalinput and video display device for the computer. In this cut-away Figureof the interactive display table, rays of light used for displaying textand graphic images are generally illustrated using dotted lines, whilerays of infrared (IR) light used for sensing objects on or just above adisplay surface 64 a of the interactive display table are illustratedusing dash lines. Display surface 64 a is set within an upper surface 64of the interactive display table. The perimeter of the table surface isuseful for supporting a user's arms or other objects, including objectsthat may be used to interact with the graphic images or virtualenvironment being displayed on display surface 64 a.

IR light sources 66 preferably comprise a plurality of IR light emittingdiodes (LEDs) and are mounted on the interior side of frame 62. The IRlight that is produced by IR light sources 66 is directed upwardlytoward the underside of display surface 64 a, as indicated by dash lines78 a, 78 b, and 78 c. The IR light from IR light sources 66 is reflectedfrom any objects that are atop or proximate to the display surface afterpassing through a translucent layer 64 b of the table, comprising asheet of vellum or other suitable translucent material with lightdiffusing properties. Although only one IR source 66 is shown, it willbe appreciated that a plurality of such IR sources may be mounted atspaced-apart locations around the interior sides of frame 62 to prove aneven illumination of display surface 64 a. The infrared light producedby the IR sources may: exit through the table surface withoutilluminating any objects, as indicated by dash line 78 a;

-   -   illuminate objects on the table surface, as indicated by dash        line 78 b; or    -   illuminate objects a short distance above the table surface but        not touching the table surface, as indicated by dash line 78 c.

Objects above display surface 64 a include a “touch” object 76 a thatrests atop the display surface and a “hover” object 76 b that is closeto but not in actual contact with the display surface. As a result ofusing translucent layer 64 b under the display surface to diffuse the IRlight passing through the display surface, as an object approaches thetop of display surface 64 a, the amount of IR light that is reflected bythe object increases to a maximum level that is achieved when the objectis actually in contact with the display surface.

A digital video camera 68 is mounted to frame 62 below display surface64 a in a position appropriate to receive IR light that is reflectedfrom any touch object or hover object disposed above display surface 64a. Digital video camera 68 is equipped with an IR pass filter 86 a thattransmits only IR light and blocks ambient visible light travelingthrough display surface 64 a along dotted line 84 a. A baffle 79 isdisposed between IR source 66 and the digital video camera to prevent IRlight that is directly emitted from the IR source from entering thedigital video camera, since it is preferable that this digital videocamera should produce an output signal that is only responsive to the IRlight reflected from objects that are a short distance above or incontact with display surface 64 a and corresponds to an image of IRlight reflected from objects on or above the display surface. It will beapparent that digital video camera 68 will also respond to any IR lightincluded in the ambient light that passes through display surface 64 afrom above and into the interior of the interactive display (e.g.,ambient IR light that also travels along the path indicated by dottedline 84 a).

IR light reflected from objects on or above the table surface may be:

-   -   reflected back through translucent layer 64 b, through IR pass        filter 86 a and into the lens of digital video camera 68, as        indicated by dash lines 80 a and 80 b; or    -   reflected or absorbed by other interior surfaces within the        interactive display without entering the lens of digital video        camera 68, as indicated by dash line 80 c.

Translucent layer 64 b diffuses both incident and reflected IR light.Thus, as explained above, “hover” objects that are closer to displaysurface 64 a will reflect more IR light back to digital video camera 68than objects of the same reflectivity that are farther away from thedisplay surface. Digital video camera 68 senses the IR light reflectedfrom “touch” and “hover” objects within its imaging field and produces adigital signal corresponding to images of the reflected IR light that isinput to PC 20 for processing to determine a location of each suchobject, and optionally, the size, orientation, and shape of the object.It should be noted that a portion of an object (such as a user'sforearm) may be above the table while another portion (such as theuser's finger) is in contact with the display surface. In addition, anobject may include an IR light reflective pattern or coded identifier(e.g., a bar code) on its bottom surface that is specific to that objector to a class of related objects of which that object is a member.Accordingly, the imaging signal from digital video camera 68 can also beused for detecting each such specific object, as well as determining itsorientation, based on the IR light reflected from its reflectivepattern, in accord with the present invention. The logical stepsimplemented to carry out this function are explained below.

PC 20 may be integral to interactive display table 60 as shown in FIG.2, or alternatively, may instead be external to the interactive displaytable, as shown in the embodiment of FIG. 3. In FIG. 3, an interactivedisplay table 60′ is connected through a data cable 63 to an external PC20 (which includes optional monitor 47, as mentioned above). As alsoshown in this Figure, a set of orthogonal X and Y axes are associatedwith display surface 64 a, as well as an origin indicated by “0.” Whilenot specifically shown, it will be appreciated that a plurality ofcoordinate locations along each orthogonal axis can be employed toindicate any location on display surface 64 a.

If the interactive display table is connected to an external PC 20 (asin FIG. 3) or to some other type of external computing device, such as aset top box, video game, laptop computer, or media computer (noneshown), then the interactive display table comprises an input/outputdevice. Power for the interactive display table is provided through apower lead 61, which is coupled to a conventional alternating current(AC) line source (not shown). Data cable 63, which connects tointeractive display table 60′, can be coupled to a USB 2.0 port, anInstitute of Electrical and Electronics Engineers (IEEE) 1394 (orFirewire) port, or an Ethernet port on PC 20. It is also contemplatedthat as the speed of wireless connections continues to improve, theinteractive display table might also be connected to a computing devicesuch as PC 20 via such a high speed wireless connection, or via someother appropriate wired or wireless data communication link. Whetherincluded internally as an integral part of the interactive display, orexternally, PC 20 executes algorithms for processing the digital imagesfrom digital video camera 68 and executes software applications that aredesigned to use the more intuitive user interface functionality ofinteractive display table 60 to good advantage, as well as executingother software applications that are not specifically designed to makeuse of such functionality, but can still make good use of the input andoutput capability of the interactive display table. As yet a furtheralternative, the interactive display can be coupled to an externalcomputing device, but include an internal computing device for doingimage processing and other tasks that would then not be done by theexternal PC.

An important and powerful feature of the interactive display table(i.e., of either embodiments discussed above) is its ability to displaygraphic images or a virtual environment for games or other softwareapplications and to enable an interaction between the graphic image orvirtual environment visible on display surface 64 a and objects that areresting atop the display surface, such as an object 76 a, or arehovering just above it, such as an object 76 b. It is the ability of theinteractive display table to visually detect such objects, as well asthe user's finger or other object being moved by the user that greatlyfacilities this rich interaction.

Again referring to FIG. 2, interactive display table 60 includes a videoprojector 70 that is used to display graphic images, a virtualenvironment, or text information on display surface 64 a. The videoprojector is preferably of a liquid crystal display (LCD) or digitallight processor (DLP) type, or a liquid crystal on silicon (LCOS)display type, with a resolution of at least 640×480 pixels. An IR cutfilter 86 b is mounted in front of the projector lens of video projector70 to prevent IR light emitted by the video projector from entering theinterior of the interactive display table where the IR light mightinterfere with the IR light reflected from object(s) on or above displaysurface 64 a. A first mirror assembly 72 a directs projected lighttraveling from the projector lens along dotted path 82 a through atransparent opening 90 a in frame 62, so that the projected light isincident on a second mirror assembly 72 b. Second mirror assembly 72 breflects the projected light onto translucent layer 64 b, which is atthe focal point of the projector lens, so that the projected image isvisible and in focus on display surface 64 a for viewing.

Alignment devices 74 a and 74 b are provided and include threaded rodsand rotatable adjustment nuts 74 c for adjusting the angles of the firstand second mirror assemblies to ensure that the image projected onto thedisplay surface is aligned with the display surface. In addition todirecting the projected image in a desired direction, the use of thesetwo mirror assemblies provides a longer path between projector 70 andtranslucent layer 64 b, and more importantly, helps in achieving adesired size and shape of the interactive display table, so that theinteractive display table is not too large and is sized and shaped so asto enable the user to sit comfortably next to it.

The foregoing and following discussions describe an interactive displaydevice in the form of interactive display table 60 and 60′.Nevertheless, it is understood that the interactive display surface neednot be in the form of a generally horizontal table top. The principlesdescribed in this description of the invention suitably also include andapply to display surfaces of different shapes and curvatures and thatare mounted in orientations other than horizontal. Thus, although thefollowing description refers to placing physical objects “on” theinteractive display surface, physical objects may be placed adjacent tothe interactive display surface by placing the physical objects incontact with the display surface, or otherwise adjacent the displaysurface.

IR Images Captured With and Without a Controlled IR Source

Although interactive display table 60 (FIG. 2) employs an IR pass filter86 a, this filter only excludes non-IR light from reaching the IR videocamera. Extraneous IR light signals also should be limited orcompensated to prevent these undesired signals from interfering with thefunctioning of the interactive display table 60. FIGS. 4A-4F show aportion of an interactive display table to illustrate how unintended IRsignals can distort an IR-spectrum image of a user's hand 402 engagingan interactive surface 64 a.

FIG. 4A shows user's hand 402 partially touching and partially“hovering” over display surface 64 a. An IR imaging system can respondto and differentiate between physical objects touching the interactivedisplay surface and physical objects hovering proximate to the displaysurface, as described in commonly assigned, co-pending U.S. patentapplication Ser. No. 10/814,761, entitled “Determining Connectedness AndOffset Of 3D Objects Relative To An Interactive Surface,” which wasfiled on Mar. 31, 2004, the specification and drawings of which arehereby specifically incorporated herein by reference.

More particularly, in FIGS. 4A, 4C, and 4E, thumb 404 and middle finger408 of user's hand 402 are touching interactive display surface 64 a,while index finger 406 and ring finger 410 “hover” a short distanceabove interactive display surface 64 a, and little finger 412 hoversslightly further away from interactive display surface 64 a.

In FIG. 4A, beams of IR light 414 emanating from IR light source 66 passthrough interactive display surface 64 a and are incident on user's hand402. Beams of reflected IR light 416 pass back through interactivedisplay surface 64 a, continuing through IR band pass filter 86 a andinto IR video camera 68. The IR video camera produces a signalcorresponding to an image of the IR light entering the camera. Thisimage is a function of IR light beams 416 and any other IR light thatreach the IR video camera. In FIG. 4A, the only source of IR light is IRsource 66; there are no unintended ambient or extraneous IR lightsources in FIG. 4A. Baffle 79 prevents IR light from IR source 66 fromdirectly entering the lens of IR video camera 68.

FIG. 4B shows the resulting IR-image of user's hand 402 a produced by IRvideo camera 68 in response only to beams of IR light 416 for IR source66 that are reflected by user's hand 402. In response to beams ofreflected IR light 416, user's hand 402 a has a greater intensity orbrightness than that of the background 420 a in this image. Further,tips of thumb 404 a and middle finger 408 a have a greater intensity orbrightness than the rest of fingers 406 a, 410 a, and 412 a, and therest of hand 402 a, in the image. Tips of thumb 404 a and middle finger408 a are the only parts of hand 402 a touching display surface 64 a(FIG. 4A), so that the IR light reflected from the tips of the thumb andmiddle finger suffers less reduction due to the diffusion of interactivedisplay surface 64 a and therefore have a greater intensity than therest of hand 402 a or background 420 a. Although only two distinctintensity levels are shown in FIG. 4B, it will be appreciated, however,that the intensity of the reflected IR light will vary based on thereflectivity of a physical object and its distance from interactivedisplay surface 64 a.

In FIG. 4C, IR source 66 is deactivated. Thus, unlike the illustrationin FIG. 4A, beams of IR light 414 are not projected toward user's hand402, and there are no beams of IR light 416 reflected from the user'shand toward IR video camera 68. However, unlike the illustration of FIG.4A, FIG. 4C shows beams of IR light 422 and 424 originating beyondinteractive display surface 64 a, which are captured by IR video camera68. For example, IR beams 422 might represent IR light emanating fromany incandescent light source in the ambient environment of theinteractive display table, and IR beams 424 might represent beams of IRlight included in sunlight illuminating the ambient environment througha window (not shown). Beams of IR light 422 and 424 pass around user'shand 402 and between user's fingers 404-412, continuing through IR bandpass filter 86 a. The “backlighting” of user's hand 402 thereforeresults in an image that includes a shadow of user's hand 402 against abrighter background 420 b, as shown in FIG. 4D.

In FIG. 4D, user's hand 402 is dark—not bright and illuminated as inFIG. 4B. IR light coming only from behind user's hand 402 appears muchbrighter than the user's hand in this image, so that the user's handappears only as a shadow against brighter background 420 b. The image ofFIG. 4D is understandably like that of a person standing with the sunbehind the person, so that the person is backlit by the sun. The diffuserays of backlighting result in a rather homogeneous shadowy image of theperson against a much brighter background. As can be seen in FIG. 4D,there is no distinction between the images of finger tips of thumb 404 band middle finger 408 b and the rest of the image of hand 402 b andfingers 406 b, 410 b, and 412 b as there is in FIG. 4B. Thus, thebacklighting of user's hand with ambient IR light, at a minimum, tendsto reduce the contrast between touching and hovering digits and hand,and may substantially reduce the useful image processing of an imagethat includes user's hand 402. In general, any segmentation of theimage, i.e., labeling each pixel as being part of the user's hand ornot, or any pixel as part of an object placed on the interactive displaysurface or not is more difficult under these conditions. The ambientlight levels can easily match those of the reflected IR light from theIR light source. In general, it is possible to read the surfaceappearance of any object (since the ambient light does not impact thelight returned from objects if they are opaque), the it is not possibleto rely on contours and shapes, or on the absolute pixel intensities inan image as an indication of what object is in contact with theinteractive display surface.

In FIG. 4E, IR source 66 is once again activated as in FIG. 4A. Thistime, however, ambient IR sources are also active, so that, for example,ambient IR light from incandescent light 422 and sunlight 424 are alsopresent. As shown in FIG. 4E, projected beams of IR light 414 reachuser's hand 402 and fingers 404-412 from IR source 66, and reflectedbeams of IR light 416 from the hand and fingers reach IR video camera68. At the same time, ambient IR light 422 and 424, shining arounduser's hand 402 and through user's fingers 404-412, also reaches the IRvideo camera. Again, none of beams 416, 422, and 424 are blocked by IRband pass filter 86 a.

The resulting image of FIG. 4F shows that IR light beams 416 reflectedfrom the user's hand and ambient IR light beams 422 and 424 mayeffectively offset each other. As a result, contrast may besubstantially reduced, not only between the tips of user's fingers 404 cand 408 c touching interactive display 64 a and the rest of user's hand402 c and fingers 406 c, 410 c, and 412 c, but in general, betweenuser's hand 402 c and fingers 404 c-412 c, and background 420 c.Background 420 c will be bright as in FIG. 4D, and the user's fingers404 c and 408 c will be discernible, due to the reflected IR from the IRlight source. Also, user's hand 402 c will be evident as a shadowrelative to the bright ambient background. If beams of reflected IRlight 416 from the user's hand are of substantially greater intensitythan the beams of ambient IR light 422 and 424, the image of user's hand402 c and fingers 404 c-412 c still might stand out as brighter thanbackground 420 c. Active adjustment of the intensity of IR source 66would have to be made to account for beams of ambient light 422 and 424while attempting to preserve contrast between of the tips of fingers 404c and 408 c that are touching interactive display surface 64 a and thebackground illuminated by ambient IR light beams 422 and 424.

Accordingly, the present invention compensates for the effects ofundesired and/or unintended ambient sources of IR light such as externalincandescent light 422 and sunlight 424 by creating a composite set ofimaging data from which the ambient sources of IR light aresubstantially removed. Again referring to FIGS. 4B, 4D, and 4E, what isdesired is to achieve the equivalent of the image of FIG. 4B, where onlyreflected beams of IR light 416 (FIG. 4A) are received at IR videocamera 68. Unfortunately, except by substantially eliminating most ofthe ambient IR light in the environment, external sources of IR light422 and 424 will typically be included, along with reflected beams of IRlight 416, resulting in an image more like that of FIG. 4F unless thepresence of the ambient IR light is compensated. Without providingappropriate compensation, whenever there are ambient IR sources presentin the environment, the undesired background in the image of FIG. 4Dwill always be added to the desired image of IR light reflected fromobjects as in FIG. 4B, yielding an image with the problems of FIG. 4F.

While the image of FIG. 4B cannot be naturally captured excepted in anisolated setting, the environments of FIGS. 4C and 4E can be selectivelyachieved in a typical setting. FIG. 4C shows an environment in whichextraneous IR light beams 422 and 424 produce the image of FIG. 4D,which can be captured when IR source 66 is deactivated. FIG. 4E shows atypical environment that includes both reflected IR light beams 416(when IR source 66 is activated) and extraneous IR light beams 422 and424. Thus, the image of FIG. 4F can be captured when IR source 66 isselectively activated in the presence of extraneous IR light. Since theimage of FIG. 4F is the compilation of the image resulting from desiredreflected IR light of FIG. 4B and the image resulting from undesiredextraneous IR light of FIG. 4D, subtracting the image of FIG. 4D fromthe image of FIG. 4F should yield the image of FIG. 4B, therebycompensating for the extraneous or ambient IR light.

Thus, the desired image generally like that of FIG. 4B can be achievedin a typical environment by collecting two sets of image data. A firstset of image data is collected with IR source 66 activated and with anyexisting ambient or extraneous IR sources present. A second set of imagedata is also collected with IR source 66 deactivated. The second set ofimage data collected is then subtracted pixel by pixel from the firstset of image data, yielding a compensated set of image data thatincludes only reflected IR light beams generated by the IR source; theextraneous sources are effectively excluded. Ideally, the first andsecond sets of image data are collected rapidly and sequentially to, asclosely as possible, provide differently illuminated images of the sameconditions on the interactive display surface.

A preferred embodiment of the present invention does not attempt toreduce the effect of unintended IR sources based upon static ambient IRmeasurements or an initial calibration. Accordingly, at the commencementof image acquisition by the IR video camera, there is no delay for imagecalibration. In addition, the present invention adapts better to changesin ambient IR sources and light levels than a static calibration orcompensation method can. For example, if a user were asked to removephysical objects from the interactive display surface for imagecalibration then directed to continue using the interactive displaysurface, the calibration data would not take into account how thepresence of physical objects added to the interactive display surfaceafter its calibration may block signals from ambient IR sources. Thepresent invention, however, takes into consideration the IR shadows,such as that shown in FIG. 4D, which are caused by physical objectsblocking ambient IR sources. Thus, the present invention generates imagedata presenting a greater IR contrast at such points than if the IRimage captured with the IR source activated was compared to a static IRsource image that was captured without the physical object present.

Furthermore, the present invention continually appropriately accountsfor and responds to changes in ambient IR sources and IR light in theenvironment. As a result, changes in ambient IR sources, such as a roomlight being turned on or off, or the intensity of sunlight passingthrough a window changing because of variation in the weather, thepassing of time changing sun angles, or a window shade being moved, donot hamper the effectiveness of the reduction of unintended IR sourcesby the present invention. Similarly, even more transient changes inlight from ambient IR sources, for example, resulting from a personwalking between the interactive display surface and a lamp or a window,are compensated by the present invention.

System for Generating Image Data to Reduce Effects of Undesired IRSources

FIG. 5 shows a system 500 for generating composite imaging data in whichthe effects of undesired IR light sources are substantially compensated.System 500 works with interactive display surface 64 a. PC 20 (shown inFIGS. 1-3) carries out the functions of image processing and includes astorage device 516. The PC processes the images produced by the IR videocamera to detect the presence and/or movement of a user's finger 408 orother physical object disposed on or adjacent to the interactive displaysurface.

As previously described in connection with FIGS. 4A, 4C, and 4E, IRsource 66 is selectively activated to direct IR light beams 414 towardinteractive display surface 64 a. Reflected IR light beams 416, as wellas beams of ambient IR light from incandescent light 422 and sunlight424 pass through interactive display surface 64 a, continuing through IRband pass filter 86 a and are detected by a sensor 502 in IR videocamera 68.

IR video camera 68 includes an image capture synchronization output 504that produces an image capture signal at the inception of each imagecapture interval. The image capture signal is received by an IR sourcecontroller 506, which activates IR source 66 at every other imagecapture interval. For example, if the frame rate of the IR video camerais X frames per second, the effective rate for activating IR source 68is X/2 times per second. As a result, every other image frame capturedby IR video camera 68 and conveyed as a signal from data output port 508will include reflected IR beams 416 and ambient IR beams 422 and 424,while every alternative image frame will include only ambient IR beams422 and 424. This approach is currently used in a preferred embodiment,but other approaches could instead be employed. The IR video camera andthe IR source could be driven synchronously by PC 20, which could issuea command to activate or deactivate the IR source, and then immediatelyacquire a frame with the IR video camera. Or a special circuit could beprovided to maintain the synchronization. In the embodiment discussedabove, it is preferable to drive the IR video camera with a clock andsimply synch from its output pulse, as a matter of convenience.

A typical video camera captures a total of 30 frames per second.Accordingly, 15 composite sets of imaging data are captured each secondby combining 15 frames that are captured with IR source 66 activatedwith 15 frames that are captured with IR source 66 deactivated. It willbe appreciated that higher frame rates are preferable. The more framesper second that are captured, the more accurately will the framescaptured with IR source 66 activated and IR source 66 deactivatedrepresent how objects appeared at interactive display surface 64 a intime.

In one embodiment of the present invention, the images output throughdata output port 508 are stored in a data buffer memory 510. Each frameis tagged according to whether the frame was captured with the IR sourceon, or off. Tagging of frames is suitably accomplished by setting a flagto indicate whether the frames were captured with the IR source on oroff. Alternatively, frames can be tagged if the IR source is activatedonly on odd- or only on even-numbered frames. In a current preferredembodiment, the intensity of a frame is evaluated by summing the pixelintensities over the image and comparing the sum of the intensities fromone frame to the next. The frame with the greater sum of intensities isthe “on” frame. Alternatively, it would be possible to place a whitetarget at a known location on the underside of the apron surroundinginteractive display surface 64 a, so that by examining the pixelintensity in the known location, it can readily be determined if the IRsource was active in a given frame. From data buffer memory 510, framesare divided by a frame separator 512 that separates the frames as afunction of whether the IR source was activated when a frame wascaptured. Frames for which the IR source was activated are routed to apositive side of a summer 514, while frames for which the IR source 66was deactivated are routed to a negative side of summer 514, so thatvalue for each point or pixel in each frame where the only IRillumination resulted from ambient IR sources can be subtracted from thecorresponding value for that point or pixel where both IR source 66 wasactivated and ambient IR sources were present. Pixelwise subtracting the“IR source off” frames from the “IR source on” frames results incomposite imaging data having IR values representing only the reflectedIR light from IR source 66, substantially excluding the effect ofambient IR illumination.

More particularly, points of the respective images are pixelwisesubtracted at summer 514 according to Eq. (1), as follows:D(x,y)=I _(ON)(xy)−I _(OFF)(xy)  (1)where:

-   -   x,y represents a coordinate location of a point on the        interactive side of the light-permeable surface;    -   I_(ON)(x,y) represents intensity of IR light detected during the        first image capture interval at point x,y;    -   I_(OFF)(x,y) represents intensity of IR light detected during        the second image capture interval at point x,y; and    -   D(x,y) represents the net intensity of IR light at point x,y        when the intensity of the IR light captured at the point x,y        during the second image capture interval is subtracted from the        intensity of the IR light captured at the point x,y during the        first image capture interval.

Once the corresponding IR source 66 activated and IR source deactivatedframe pairs are combined at summer 514, the composite set of imagingdata generated is passed to image processing or storage, so thatcharacteristics of one or more physical objects 408 present on orproximate to the interactive display surface can be processed and theresults used by the interactive display system.

Method for Generating Image Data to Reduce Effects of Undesired IRSources

FIG. 6 is a flow diagram 600 illustrating exemplary logical steps forsubstantially eliminating or reducing the effects of undesired IRillumination according to the present invention. At a step 602, IRimaging of the interactive display surface begins. At a step 604, the IRsource is selectively synchronized with the image acquisition signalgenerated by the IR video camera (or other imaging device that mightalternatively be used). Selective IR source synchronization is desiredbecause, as previously explained, the IR source is activated only duringevery other image acquisition interval.

At a step 606, the IR source is activated. At a step 608, theinteractive display surface is imaged with the IR source turned on. At astep 610, the IR source is deactivated, and at a step 612, theinteractive display surface is imaged with the IR source turned off.Using the image frames captured at steps 608 and 612, at a step 614 theimage data captured for pairs of frames under these two differentlighting conditions are pixelwise combined, as previously described inconnection with FIG. 5 and Eq. (1) to compensate for the effect of lightfrom unintended IR sources. After the image data are combined at step614 to generate the composite data, at a step 616, the composite dataare used for image processing, such as to detect physical objects on oradjacent to the interactive display surface, or the data are stored forlater processing.

At a decision step 618, it is determined if IR imaging data arecontinuing to be captured. If so, the flow diagram loops to step 606,where the selective activation and deactivation and image capturecontinues. On the other hand, if it is determined at decision step 618that the IR imaging is complete, the flow diagram proceeds to a step 620and ends.

Although the present invention has been described in connection with thepreferred form of practicing it and modifications thereto, those ofordinary skill in the art will understand that many other modificationscan be made to the present invention within the scope of the claims thatfollow. Accordingly, it is not intended that the scope of the inventionin any way be limited by the above description, but instead bedetermined entirely by reference to the claims that follow.

1. A method for reducing an effect of an undesired infrared light sourcein an imaging system using a desired infrared light source, the methodcomprising the steps of: (a) activating the desired infrared lightsource during a first image capture interval; (b) capturing a first setof image data during the first image capture interval; (c) deactivatingthe desired infrared light source during a second image captureinterval; (d) capturing a second set of image data during the secondimage capture interval; and (e) producing a composite set of image databy subtracting from first values in the first set of image data,corresponding second values in the second set of image data.
 2. Themethod of claim 1, further comprising the step of controlling activationof the desired infrared light source with an image capture device, suchthat an image capture signal generated by the image capture devicecauses the desired infrared light source to be activated during thefirst image capture interval and deactivated during the second imagecapture interval.
 3. The method of claim 2, further comprising the stepsof: (a) positioning the desired infrared light source on a first side ofa light-permeable surface to direct infrared light on a physical objectdisposed adjacent an opposite side of the light-permeable surface; and(b) positioning the image capture device on the first side of alight-permeable surface to capture infrared light that was produced bythe desired infrared light source and reflected by the physical object.4. The method of claim 3, wherein the first values in the first set ofimage data represent an intensity of infrared light captured for aplurality of points across the first side of the light-permeable surfaceduring the first image capture interval while the desired infrared lightsource was activated, and the second values in the set of image datarepresent an intensity of infrared light captured for the plurality ofpoints across the first side of the light-permeable surface during thesecond image capture interval while the desired infrared light sourcewas deactivated.
 5. The method of claim 4, wherein the subtracting fromthe first values in the first set of image data corresponding secondvalues in the second set of image data is defined by:D(x,y)=I _(ON)(x,y)−I _(OFF)(x,y) where: (a) x,y represents a coordinatelocation of a point on the first side of the light-permeable surface;(b) I_(ON)(x,y) represents an intensity of infrared light detectedduring the first image capture interval at the point xy; (c)I_(OFF)(x,y) represents an intensity of infrared light detected duringthe second image capture interval at the point xy; and (d) D(x,y)represents a net intensity of infrared light at the point x,y when theintensity of the infrared light captured at the point x,y during thesecond image capture interval is subtracted from the intensity of theinfrared light captured at the point x,y during the first image captureinterval.
 6. The method of claim 3, further comprising the step ofcommunicating the composite set of image data to an image processingsystem.
 7. The method of claim 6, further comprising the step of usingthe infrared light reflected by the physical object to recognize acharacteristic of the physical object.
 8. The method of claim 3, furthercomprising the step of positioning a projector on the first side of thelight-permeable surface such that light from the projector directed ontothe light-permeable surface is used to present images visible on theopposite side of the light-permeable surface with which the physicalobject can interact.
 9. A method for reducing an effect of an undesiredinfrared light source in an imaging system using a desired infraredlight source, comprising the steps of: (a) selectively coordinatingactivation of the desired infrared light source with an image signalgenerated by an image capture device; (b) activating the desiredinfrared light source when the image signal indicates initiation of afirst image capture interval; (c) capturing a first set of image dataduring the first image capture interval; (d) deactivating the desiredinfrared light source when the image signal indicates initiation of asecond image capture interval; (e) capturing a second set of image dataduring the second image capture interval; (f) generating a composite setof image data by subtracting from first values in the first set of imagedata, corresponding second values in the second set of image data; and(g) communicating the composite set of image data to an image processingsystem.
 10. The method of claim 9, further comprising the steps of: (a)positioning the desired infrared light source on a first side of alight-permeable surface to direct infrared light on a physical objectdisposed adjacent an opposite side of the light-permeable surface; and(b) positioning the image capture device on the first side of alight-permeable surface to capture infrared light from the desiredinfrared light source that is reflected by the physical object.
 11. Themethod of claim 10, wherein the first values in the first set of imagedata represent an intensity of infrared light captured for a pluralityof points across the first side of the light-permeable surface duringthe first image capture interval while the desired infrared light sourcewas activated, and the second values in the set of image data representan intensity of infrared light captured for a correspondingly pluralityof points across the first side of the light-permeable surface duringthe second image capture interval while the desired infrared lightsource was deactivated.
 12. The method of claim 11, wherein the step ofsubtracting from first values in the first set of image datacorresponding second values in the second set of image data is definedby:D(x,y)=I _(ON)(x,y)−I _(OFF)(x,y) where: (a) x,y represents a coordinatelocation of a point on the first side of the light-permeable surface;(b) I_(ON)(x,y) represents an intensity of infrared light detectedduring the first image capture interval at the point x,y; (c)I_(OFF)(x,y) represents an intensity of infrared light detected duringthe second image capture interval at the point x,y; and (d) D(x,y)represents a net intensity of infrared light at the point x,y when theintensity of the infrared light captured at the point x,y during thesecond image capture interval is subtracted from the intensity of theinfrared light captured at the point x,y during the first image captureinterval.
 13. The method of claim 10, further comprising the step ofusing the infrared light reflected by the physical object to recognize acharacteristic of the physical object.
 14. The method of claim 10,further comprising the step of positioning a projector on the first sideof the light-permeable surface such that light from the projector isdirected onto the light-permeable surface to present images visible onthe opposite side of the light-permeable surface with which the physicalobject can interact.
 15. A method for capturing image data for aphysical object disposed on or adjacent to a light-permeable surfaceusing a desired infrared light source and an image capture device,comprising the steps of: (a) positioning the desired infrared lightsource on a first side of the light-permeable surface to direct infraredlight toward the physical object; (b) positioning the image capturedevice on the first side of a light-permeable surface to captureinfrared light cast by the desired infrared light source and reflectedby the physical object; (c) activating the desired infrared light sourcewhen the image signal indicates initiation of a first image captureinterval; (d) capturing a first set of image data during the first imagecapture interval; (e) deactivating the desired infrared light sourcewhen the image signal indicates initiation of a second image captureinterval; (f) capturing a second set of image data during the secondimage capture interval; and (g) generating a composite set of image databy subtracting from first values in the first set of image data,corresponding second values in the second set of image data.
 16. Themethod of claim 15, further comprising the step of controllingactivation of the desired infrared light source with the image capturedevice, such that an image capture signal generated by the image capturedevice causes the desired infrared light source to be activated duringthe first image capture interval and deactivated during the second imagecapture interval.
 17. The method of claim 15, wherein the first valuesin the first set of image data represent an intensity of infrared lightcaptured for a plurality of points across the first side of thelight-permeable surface during the first image capture interval whilethe desired infrared light source was activated, and the second valuesin the set of image data represent an intensity of infrared lightcaptured for the plurality of points across the first side of thelight-permeable surface during the second image capture interval whilethe desired infrared light source was deactivated.
 18. The method ofclaim 17, wherein the step of subtracting from first values in the firstset of image data, corresponding second values in the second set ofimage data is defined by:D(x,y)=I _(ON)(x,y)−I _(OFF)(x,y) where: (a) xy represents a coordinatelocation of a point on the first side of the light-permeable surface;(b) I_(ON)(x,y) represents an intensity of infrared light detectedduring the first image capture interval at a point x,y; (c) I_(OFF)(x,y)represents an intensity of infrared light detected during the secondimage capture interval at a point x,y; and (d) D(x,y) represents a netintensity of infrared light at a point x,y when the intensity of theinfrared light captured at the point x,y during the second image captureinterval is subtracted from the intensity of the infrared light capturedat the point x,y during the first image capture interval.
 19. The methodof claim 15, further comprising the step of communicating the compositeset of image data to an image processing system.
 20. The method of claim19, further comprising the step of using the infrared light reflected bythe physical object to recognize a characteristic of the physicalobject.
 21. The method of claim 15, further comprising the step ofpositioning a projector on the first side of the light-permeable surfacesuch that the projector directs light onto the light-permeable surfaceto present images visible on the opposite side of the light-permeablesurface, with which the physical object can interact.
 22. A system forproviding input to an application that is being executed, comprising:(a) a light-permeable surface having a processing side and aninteractive side, the interactive side being configured enable aphysical object to be disposed on or adjacent to the interactive side,the processing side being opposite to the interactive side; (b) adesired infrared light source disposed on the processing side of thelight-permeable surface, the desired infrared light source selectivelyemitting infrared light that is transmitted through the light-permeablesurface to the interactive side and reflected back through thelight-permeable surface by the physical object that is disposed on oradjacent to the interactive side of the light-permeable surface; (c) animage capture device disposed on the processing side of thelight-permeable surface, the image capture device sensing infrared lightpassing through the light permeable surface and imaging the interactivedisplay surface to detect the physical object and its location; (d) aprocessor in communication with the desired infrared light source andthe image capture device; and (e) a memory in communication with theprocessor, the memory storing data and machine instructions that causethe processor to carry out a plurality of functions, including: (i)activating the desired infrared light source during a first imagecapture interval; (ii) capturing a first set of image data during thefirst image capture interval with the image capture device; (iii)deactivating the desired infrared light source during a second imagecapture interval; (iv) capturing a second set of image data during thesecond image capture interval with the image capture device; and (v)generating a composite set of image data by subtracting from firstvalues in the first set of image data corresponding second values in thesecond set of image data, an effect of infrared light captured by theimage capture device but not emitted by the desired infrared sourcebeing substantially eliminated in the composite set of data.
 23. Thesystem of claim 22, wherein the first values in the first set of imagedata represent an intensity of infrared light captured by the imagecapture device for a plurality of points across the first side of thelight-permeable surface during the first image capture interval whilethe desired infrared light source was activated, and the second valuesin the set of image data represent an intensity of infrared lightcaptured by the image capture device for the plurality of points acrossthe first side of the light-permeable surface during the second imagecapture interval while the desired infrared light source wasdeactivated.
 24. The system of claim 22, wherein the machineinstructions stored in the memory further cause the processor tosubtract from first values in the first set of image data, correspondingsecond values in the second set of image data, as defined by:D(x,y)=I _(ON)(x,y)−I _(OFF)(x,y) where: (a) x,y represents a coordinatelocation of a point on the first side of the light-permeable surface;(b) I_(ON)(xy) represents an intensity of infrared light detected by theimage capture device during the first image capture interval at thepoint x,y; (c) I_(OFF)(x,y) represents an intensity of infrared lightdetected by the image capture device during the second image captureinterval at the point x,y; and (d) D(x,y) represents a net intensity ofinfrared light at point x,y when the intensity of the infrared lightcaptured at the point x,y during the second image capture interval issubtracted from the intensity of the infrared light captured at thepoint x,y during the first image capture interval.
 25. The system ofclaim 22, wherein the machine instructions stored in the memory furthercause the processor to use the infrared light reflected by the physicalobject to recognize a characteristic of the physical object.
 26. Thesystem of claim 22, further comprising a projector positioned on theprocessing side of the light-permeable surface such that light from theprojector is directed onto the light-permeable surface and used topresent images visible on the interactive side of the light-permeablesurface with which the physical object can interact.
 27. A system forproviding input to an application that is being executed, comprising:(a) a light-permeable surface having a processing side and aninteractive side, the interactive side being configured to enable aphysical object to be disposed on or adjacent to the interactive side,the processing side being opposite to the interactive side; (b) an imagecapture device disposed on the processing side of the light-permeablesurface, the image capture device sensing infrared light passing throughthe light permeable surface and imaging the interactive display surfaceto detect the physical object and its location, the image capture devicebeing configured to generate an image capture signal; (c) a desiredinfrared light source disposed on the processing side of thelight-permeable surface, the desired infrared light source selectivelyemitting infrared light that is transmitted through the light-permeablesurface to the interactive side and reflected back through thelight-permeable surface by the physical object that is disposed on oradjacent to the interactive side of the light-permeable surface; (d) aninfrared light source controller configured to: (i) receive the imagecapture signal; and (ii) selectively actuate the desired infrared lightsource in response to receipt of the image capture signal, such that thedesired infrared light source is activated on receipt of a first imagecapture signal indicating a first image capture interval and deactivatedon receipt of a second image capture signal indicating a second imagecapture interval; (e) a processor in communication infrared light sourceand the image capture device; and (f) a memory in communication with theprocessor, the memory storing data and machine instructions that causethe processor to carry out a plurality of functions, including: (i)capturing a first set of image data with the image capture device duringthe first image capture interval; (ii) capturing a second set of imagedata with the image capture device during the second image captureinterval; and (iii) generating a composite set of image data bysubtracting from first values in the first set of image data,corresponding second values in the second set of image data, any effectof extraneous infrared light captured by the image capture device beingsubstantially compensated in the composite set of image data.
 28. Thesystem of claim 27, wherein the first values in the first set of imagedata represent an intensity of infrared light captured by the imagecapture device for a plurality of points across the first side of thelight-permeable surface during the first image capture interval whilethe desired infrared light source was activated, and the second valuesin the set of image data represent an intensity of infrared lightcaptured for the plurality of points across the first side of thelight-permeable surface during the second image capture interval whilethe desired infrared light source was deactivated.
 29. The system ofclaim 28, wherein the machine instructions stored in the memory furthercause the processor to subtract from first values in the first set ofimage data, corresponding second values in the second set of image data,as defined by:D(x,y)=I _(ON)(x,y)−I _(OFF)(x,y) where: (a) x,y represents a coordinatelocation of a point on the first side of the light-permeable surface;(b) I_(ON)(xy) represents an intensity of infrared light detected duringthe first image capture interval at the point x,y; (c) I_(OFF)(x,y)represents an intensity of infrared light detected during the secondimage capture interval at the point x,y; and (d) D(x,y) represents a netintensity of infrared light at the point x,y when the intensity of theinfrared light captured at the point x,y during the second image captureinterval is subtracted from the intensity of the infrared light capturedat the point x,y during the first image capture interval.
 30. The systemof claim 27, wherein the machine instructions stored in the memoryfurther cause the processor to use the infrared light reflected by thephysical object to recognize a characteristic of the physical object.31. The system of claim 27, further comprising a projector positioned onthe processing side of the light-permeable surface such that lightproduced by the projector is directed onto the light-permeable surfaceand used to present images visible on the interactive side of thelight-permeable surface with which the physical object can interact.